1. Field of the Invention
The present invention relates to a shutter time control system for flash photography of the type which automatically switches the shutter time of a camera to that to a flash made in response to a charging signal of a main capacitor.
2. Description of the Prior Art
As a system to automatically change over a shutter time of a camera to that for flash photographing (for example 1/60 second) as a photographing mode is changed over from a natural light photographing to flash photographing, such system has already been proposed that when a main capacitor for accumulation of flash light energy of a flash light device is charged to a prescribed voltage, the same is detected and the charging signal is transmitted to a shutter time control circuit of a camera from the flash light device and said shutter time control circuit is made to respond to said charging signal so that said shutter time control circuit is controlled automatically to that suited to flash photographing. But said conventional system has the shutter time control circuit changed over from the one for day light into the one for flash photographing by a charging completion signal, therefore it has such shortcomings that when the flash light device emits light while the shutter time is controlled by the shutter time control circuit for flash photographing and charging signal is eliminated, the shutter time control circuit is changed over from the shutter time control circuit for flash photographing again into the shutter time control circuit for day light, and the shutter time actually secured at a time of flash photographing receives the effect of both circuits of the shutter time control circuit for day light and the shutter time control circuit for flash photographing, thus the shutter time suited to a flash photographing can not be secured. That is, since the shutter time is changed over from the shutter time control circuit for day light to that for flash light by charging signal in a conventional flash photographing shutter time control system as mentioned above, the relationship between the output Vc of a time constant circuit to control the shutter time for flash photographing at a time of flash photographing and the shutter time t is as shown in FIG. 1. In said drawing Vs shows the threshold level of a Schmidt circuit of a known shutter control circuit which gives a shutter closing signal to such shutter closing member as a shutter follower screen etc. in response to the output of the time constant circuit, also t.sub.1 shows a shutter time necessary for flash photographing while t.sub. 2 shows a time at which such shutter opening member as a shutter leading screen etc. fully opens an aperture part, wherein the opening member is driven by shutter release and the aperture part is fully opened at the time t.sub.2. But, as a shutter time necessary for flash photographing can not be obtained if the shutter closing member is driven at said time, said threshold level Vs of the Schmidt circuit is made higher than the output of the time constant circuit at the time t.sub.2, and the Schmidt circuit has not outputted a shutter closing signal yet, at said time. Also while a synchronizing contact is closed by a known method as a shutter is fully opened at the time t.sub.2 and a flash tube emits flash light, the output of a main capacitor for flash light energy accumulation is lowered as the flash light is generated and said charging signal disappears. Therefore, as the shutter time control circuit is changed over from flash light to day light immediately after the time t.sub.2, the output of the time constant circuit after the time t.sub.2 will be controlled by shutter time information for day light, thus said conventional system has such shortcoming that if it is of a control circuit to preset a shutter time, the output of a time constant circuit varies as shown in B or C by said set shutter time, and the time reaching the threshold level will be different from the shutter time t.sub.1 suited to flash photographing, such as t.sub.3 and t.sub.4, etc.